The present invention generally relates to polishing a surface of a workpiece. More particularly, the invention relates to improved methods and apparatus for detecting the presence of a wafer in a carrier.
Chemical mechanical polishing or planarizing a surface of an object may be desirable for several reasons. For example, chemical mechanical polishing is often used in the formation of microelectronic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. Chemical mechanical polishing may also be used to form microelectronic features. For example, a conductive feature such as a metal line or a conductive plug may be formed on a surface of a wafer by forming trenches and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the surface of the wafer using chemical mechanical polishing, leaving the vias and trenches filled with the conductive material.
A typical chemical mechanical polishing apparatus suitable for planarizing the semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to a wafer during the polishing process; a polishing compound such as a slurry containing abrasive particles and chemicals to assist removal of material from the surface of the wafer; and a polishing surface such as a polishing pad. In addition, the polishing apparatus may include an integrated wafer cleaning system and/or an automated load and unload station to facilitate automatic processing of the wafers.
A wafer surface is generally polished by moving the surface of the wafer to be polished relative to the polishing surface in the presence of the polishing compound. In particular, the wafer is placed in the carrier such that the surface to be polished is placed in contact with the polishing surface and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
The wafer is typically placed in the carrier by a robot or other mechanical means. Conventional carriers have a retaining ring surrounding either an elastic surface supported by a rigid pressure plate or a membrane supported by one or more pressurized plenums. The retaining ring and elastic surface or membrane form a pocket for receiving the wafer. A vacuum is typically applied to the back surface of the wafer through one or more apertures in the rubber surface or membrane for holding the wafer in the carrier.
Occasionally, the wafer is improperly positioned in the carrier or the vacuum is insufficient to properly retain the wafer in the carrier. It is important to verify the wafer is properly held before the start of the polishing process to prevent damage to the wafer or the polishing tool. The aperture(s) allow the vacuum level to be checked to determine if the wafer is present. If the wafer is properly held by the carrier, the wafer will block the aperture(s) thereby maintaining the vacuum. However, if the wafer becomes dislodged, the wafer will not block the aperture(s) and the vacuum level will not be maintained.
After processing, it is typically desirable to remove the wafer from the carrier to allow processing of subsequent wafers. The wafer often sticks to the rubber surface or membrane and may not be easily dislodged from the carrier. The aperture provides a path for pressurized air to be used to blow the wafer off the carrier.
Applicants have noticed several problems with using the aperture of the prior art. The aperture creates a point of nonuniform pressure on the back surface of the wafer. This nonuniform pressure on the back surface of the wafer could result in a nonuniform removal of material on the front surface of the wafer. Another problem noticed by the applicants is that the pressurized air used to dislodge the wafer may impart particles previously sucked into the air lines while holding another wafer on the back surface of the present wafer. The pressurized air may also dry particles already on the back surface of the wafer thereby making the particles more difficult to remove. The particles can contaminate the wafer and hinder further processing of the wafer.
What is needed is an apparatus and method for verifying the presence of a wafer in a carrier that does not hinder the processing of the wafer and that does not impart or adhere particles to the wafer.
The present invention provides improved methods and apparatus for chemical mechanical polishing of a surface of a workpiece that overcome many of the shortcomings of the prior art. While the ways in which the present invention addresses the drawbacks of the now-known techniques for chemical mechanical polishing will be described in greater detail hereinbelow, in general, in accordance with various aspects of the present invention.
A carrier may be used to retain a wafer during a polishing or planarizing process of a front surface of the wafer. The carrier may advantageously be configured to also detect the presence of the wafer in the carrier. The carrier includes a body having a shallow recess or cavity. The body provides structural support for the carrier. The body may also be connected to the rest of the tool via a shaft for pressing and possibly rotating the body. In one particular embodiment, the shaft is connected to a carousel apparatus for transporting the carrier from one station to another station within the tool.
A membrane may be positioned over the cavity to form an initial plenum between the body and the membrane. The membrane is used to support the back surface of the wafer during the polishing process. The membrane is preferably a flexible resilient film comprising an elastic surface of uniform thickness. However, the membrane may have variations in thickness or even a notch for receiving a portion of the sensor if this improves the detection capability of the sensor or the polishing process.
A sensor may be positioned within the initial plenum for detecting the presence of the wafer. The sensor may be, for example, an inductive or mechanical sensor, but is preferably a capacitive sensor mounted to the body. The size of a capacitive sensor positioned within the plenum may be reduced by positioning the sensor""s amplifier outside of the plenum. The sensor is preferably connected to a control system that may act upon the information acquired from the sensor.
In a preferred embodiment, one or more dividers, preferably annular ribs, may extend from the body to the membrane to divide the initial plenum into a plurality of plenums. The dividers or annular ribs allow the carrier to have a plurality of zones that may be used to apply different urging forces on different areas of the back surface of the wafer. These multizone carriers greatly improve the flexibility of the polishing process. For even greater process flexibility, independently controllable plenums may be created behind the annular ribs to control the urging forces on different areas of the back surface of the wafer adjacent to the ribs.
One or more pumps having one or more pressure regulators may be used to pressurize the initial plenum, plurality of plenums in the carrier or plenums behind the ribs through pathways during a wafer detection process or the polishing process. The pumps with pressure regulators and pathways to the plenum(s) may be configured in a variety of ways, but they are preferably configured so that the pressure within each plenum is independently controllable.
A platen supporting a polishing surface may also be used during the polishing process of the wafer. A manifold may be positioned beneath the platen to distribute a fluid across the platen to the polishing surface during the polishing process. The fluid may be uniformly distributed or distributed in a desired manner to improve the polishing process.
A motion generator may be connected to the platen for generating relative motion between the wafer and the polishing surface. The motion generator may, for example, orbit, rotate or linearly move the polishing surface.
In operation it may be determined whether a wafer is retained by a carrier by positioning the back surface of the wafer against the membrane of the carrier and sensing through the membrane, preferably via a capacitive sensor, whether the wafer is present. If the wafer is detected, the wafer is properly retained and the polishing process may continue. However, if the wafer is not detected, the wafer has not been properly retained and the polishing process may be terminated.
A more sophisticated method of determining whether a wafer is retained by a carrier may be used with a multizone carrier. The back surface of the wafer is once again positioned against the membrane of the carrier. A first pressure may be applied in a first plenum sufficient to retain the wafer by creating a partial vacuum between the membrane and the wafer. The partial vacuum is created by the pressure pulling the membrane away from the back surface of the wafer in the region of the first plenum. The sensor may be used at this time to detect the presence of the wafer through the membrane, but preferably, a second pressure may be applied in a second plenum sufficient to urge the membrane away from the sensor. The presence of the membrane adjacent the sensor may thus be used to detect the presence of the wafer. If the membrane has moved away from the sensor, the wafer is not holding the membrane against the sensor and thus has not been properly retained. If the membrane remains adjacent to the sensor, the wafer has been properly retained. Care should be taken to make sure the second pressure in the second plenum is sufficient to urge the membrane away from the sensor if the wafer is not present, but insufficient to dislodge the wafer from the carrier if the wafer is present. The pressure within each plenum may need to be optimized depending on, for example, the size and location of the plenums and the flexibility of the membrane. If the membrane is touching or near the sensor, the wafer is properly retained and the polishing process may continue. However, if the membrane has been urged away from the sensor in the second plenum, the wafer has not been properly retained and the polishing process may be terminated.